The present invention relates generally to apparatus for non-contact printing of indicia on a continuous length elongated member, such as an insulated wire and the like, and more particularly to a new and improved method and apparatus for guiding such an elongated member longitudinally past an ink jet printing station to enable high quality printed images to be placed on the member.
It is a common practice in identifying insulated wire and the like to apply identifying indicia on the exterior surface of the outer insulation layer or sheath. The identifying indicia may comprise relatively simple coding such as a particular wire gauge or size, for example, A.W.G. 16. In many applications, however, multiple insulated electrical wires are grouped to form a wire harness which may be installed in a circuit employing a number of multiple-wire harnesses interconnecting plugs, connectors and other terminal components. In such applications it is particularly important to be able to identify and distinguish each wire in the same or similar wire harnesses. To meet this need, multiple character alpha-numeric codes are applied to the individual insulated wire strands, or to the exterior surface of a multi-conductor longitudinally twisted wire group. Prior practices have included cold printing and hot stamping the alpha-numeric identification codes onto the insulating layers or sheaths of the electrical conductors, and securing printed tapes or sleeve tags on the wires at spaced intervals. These techniques have significant disadvantages such as requiring intermittent movement of the wire so that it is stationary during printing or application of identifying tags. In addition, the prior techniques are relatively slow and in the case of direct printing onto the insulated wires, often result is smudged or blurred identifying indicia. Moreover, it has been found that these prior techniques do not work satisfactorily with twisted, straight-stranded or braided wires because of their irregular exterior surfaces.
In an attempt to overcome the aforedescribed problems encountered in applying identifying indicia to insulated conductor wires and the like having three dimensional outer surfaces, ink jet printing techniques have been employed to apply alpha-numeric characters to electrical wires and other continuous elongated flexible members. See, for example, U.S. Pat. No. 4,029,006. A significant drawback in the methodology disclosed in U.S. Pat. No. 4,029,006 is that the elongated member is guided past an ink jet printing head by tubular guides disposed on the upstream and downstream sides of the printing head. Since the ink jet printing head projects electrostatically charged ink droplets onto the outer surface of the passing elongated conductor wire to form identifying indicia, passage of the imprinted wire through a tubular guide on the downstream side of the print head in a manner to physically guide the wire may result in smudging or blurring of the printed indicia. Further, the elongated wire has a tendency to rotate about its longitudinal axis as it is fed from a wound supply reel or spool, with the result that the printed indicia or characters may not be in proper longitudinal alignment on the outer surface of the wire, thus inhibiting easy reading of the indicia during field installation.
A more recent technique for applying indicia to the outer surface of an elongated generally cylindrical member, such as a continuous length insulated conductor wire, by ink jet printing employs a wire guide mechanism having pairs of rollers upstream and downstream of a ink jet print head. The rollers define nips through which the wire passes to guide the wire past the print head. While this technique provides improved performance over prior wire handling mechanisms utilized with ink jet printing processes, an apparatus or mechanism which enables high quality ink jet printing of indicia onto the three-dimensional external surface of an elongated member, such as an insulated electrical conductor wire, by maintaining the elongated member in a relatively constant rotational orientation as it moves longitudinally past the ink jet print head without smudging or smearing would provide a significant improvement over prior wire handling mechanisms.